Coherent optical communication systems have been developed that provide distinct advantages over more traditional direct detection schemes. In a coherent optical communication receiver, the optical phase and amplitude of a transmitted optical signal can be detected, thus enabling the use of multilevel modulation schemes to increase optical fiber spectral efficiency. Coherent detection provides another advantage over direct detection in that linear impairments of the transmitted optical signal can be compensated for in the receiver using digital filters and other known digital compensation techniques.
Some important linear impairments that can affect the performance of optical communication systems include two forms of signal distortion resulting from chromatic dispersion (CD) and polarization mode dispersion (PMD) of the optical signal. The transmitted optical signal has a finite spectral width such that the spectral components may be transmitted at different frequencies. Chromatic dispersion is a result of the different spectral components propagating at different speeds along the fiber, resulting in an undesirable temporal spreading of the optical signal. PMD occurs due to the different polarization modes of the optical signal propagating along the fiber at different speeds and is affected by environmental effects and asymmetries in the optical fiber, which are often random, unpredictable and can vary over time. Due to the random and unpredictable nature of PMD, PMD cannot be easily determined or compensated for in a conventional optical communication system.
Optical communication systems generally include a number of links of optical fibers and optical system components, each contributing to the overall chromatic dispersion of received signals. The effects of chromatic dispersion are linear and deterministic and can be more easily compensated for than the effects of PMD. One conventional method of compensating for chromatic dispersion in an optical communication system includes the use of dispersion compensation fibers (DCFs) or other components that compensate chromatic dispersion optically. In practice, however, implementing such DCFs and other known dispersion compensating techniques and components in the optical communication system undesirably increases the size of the system, and can be cumbersome and expensive. Additionally, such dispersion compensation components can undesirably limit the power and spectral efficiency of the optical communication system.
Recent advances in coherent receiver technology allow for compensation of linear transmission impairments, such as chromatic dispersion, by utilizing digital compensation in the receiver instead of performing optical compensation using DCFs, such as those described above. One challenge, however, is that this type of coherent optical receiver should be designed to compensate for a large chromatic dispersion, such as values as high as 51,000 ps/nm. Another challenge is that since the received signal is affected by both chromatic dispersion and PMD, it is more difficult to estimate the effects of either one. In the conventional systems discussed above, the chromatic dispersion is commonly compensated for first, or otherwise considered to be negligible, so that the PMD can be more accurately estimated at the receiver. In a system with potentially large chromatic dispersion and PMD, it is very difficult to accurately and efficiently estimate these effects in the receiver, thus seriously degrading performance of the optical communication system.
Some known methods for dealing with these impairments utilize a preset or adaptive filter to compensate for a known chromatic dispersion in the receiver. These solutions, however, are limited in that in many systems, especially switched systems in which the signal may travel via one of several different links, the chromatic dispersion effects cannot be easily known or determined in advance because each link scenario has a different chromatic dispersion, or the adaptive filter cannot be easily or efficiently updated.
In other known systems, where the chromatic dispersion is unknown, the chromatic dispersion may be estimated by iteratively scanning a range of chromatic dispersion values for the received signal until some control loops in the receiver are working, e.g., automatic gain control (AGC) and clock recovery loop. The scanning is performed by estimating a first value for the chromatic dispersion (such as 1000 ps/nm), calculating a chromatic dispersion coefficient, evaluating a receiver circuit response using the calculated chromatic dispersion coefficients in a compensation filter, then revising the estimate if necessary. The range of the estimated chromatic dispersion values should be relatively narrow to minimize estimation error and ensure that the control loops work, that is, to ensure a clock phase detector has satisfactory sensitivity. For example, the estimated chromatic dispersion value in each step should differ by 1000 ps/nm or less per step. To compensate for a chromatic dispersion of up to 50,000 ps/nm (from the above example), the scan may require approximately 50 steps or iterations. For each iteration, the step of calculating the chromatic dispersion coefficients results in even greater complexity. Additionally, there is added delay in such systems due to the time required for the acquired signal to be allocated for the chromatic dispersion scanning steps discussed above. Another disadvantage of such methods is that the chromatic dispersion cannot accurately be estimated until the clock recovery loop is locked, but the clock recovery loop cannot lock without the chromatic dispersion value first being determined, leading to added delay in acquiring a desired sensitivity of the clock recovery loop. Such methods of scanning for the chromatic dispersion value lead to an undesired increase in processing time and undue complexity in the receiver.
There are known methods that include increasing the efficiency of such scanning methods described above by employing adaptive techniques to reduce the processing time and minimize the chromatic dispersion estimation error. For instance, one known method proposes an adaptive algorithm for determining an equalizer metric in a frequency domain equalizer (FDE) to estimate the chromatic dispersion from the received signal. In another known method, each of the transfer functions for a range of predetermined chromatic dispersion values are determined and pre-stored in a look-up table to reduce complexity at the receiver. These methods, however, are still too inefficient for a receiver when there is large chromatic dispersion in the optical communication system.
Furthermore, for optical communication systems with large PMD, the above receiver techniques do not provide satisfactory sensitivity with respect to the PMD effects, thus resulting in poor system performance. Specifically, the chromatic dispersion estimation methods discussed above fail to discriminate between actual chromatic dispersion effects and the effects of the second and higher order components of PMD present in the optical communication system. Second order PMD is characterized as the derivative of the first order PMD with respect to frequency. Because second order PMD is a function of frequency, it mimics chromatic dispersion. If the estimated chromatic dispersion in these known methods includes the effects of second order PMD, the receiver sensitivity can be seriously corrupted.
There is a need, therefore, for an efficient, yet robust method of estimating the chromatic dispersion of a modulated and polarization-multiplexed signal transmitted over a fiber optic medium that is also insensitive to PMD effects. Thus, it is desirable to implement a method which includes estimating the chromatic dispersion directly from the received signal, without iterative scanning for the chromatic dispersion value, that distinguishes the unfavorable chromatic dispersion effects from those resulting from PMD. It is also desirable to implement a method in which the chromatic dispersion is accurately estimated prior to and/or independent from locking a clock recovery loop in the receiver.